1. Field of the Invention
The present invention relates to an oscillating circuit, and more particularly to a low power micropower RC oscillator that has stable oscillation frequency characteristics with varying temperature and capable of variably changing oscillation frequency by adjusting a resistance value.
2. Description of the Related Art
Oscillating circuits are commonly used as devices for providing a clock signal or a timing signal to electronic circuits such as a micro-processor, a micro-controller, a flip-flop circuit and a latch circuit. They are widely applied in industrial electronics. A crystal oscillating circuit provides an accurate and stable reference frequency.
However, in many applications, such a high-quality reference frequency is not required and since a mass productivity should be considered, an RC-type oscillator can be used as a low-cost clock signal source or a timing signal source. Also, an RC oscillator generates a variable frequency by changing the resistance R or capacitance C and avoids the use of inductors, which are difficult to fabricate on integrated circuits.
For various electronic circuit applications using a battery, such as mobile communication devices or portable electronic devices, very low current dissipation is required. Therefore, the micropower RC oscillator is suitable in those applications requiring low power dissipation since low power dissipation leads to longer battery life.
A micropower RC oscillator is disclosed in U.S. Pat. No. 5,570,067 (granted to Stuart B. Shacter) entitled xe2x80x9cMICROPOWER RC OSCILLATOR HAVING HYSTERESIS PRODUCED BY SWITCHING CURRENT SOURCES TO A TRANSISTORxe2x80x9d.
According to the above-mentioned U.S. patent, the RC oscillator employs a plurality of amplifiers and varies a threshold voltage thereof to obtain a hysteresis instead of using an RC circuit, a charge/discharge circuit, a general comparator and a general hysteresis circuit to change a reference voltage of the comparator. With this configuration, the oscillation frequency receives only a few effects by an internal delaying device, the RC oscillator can be operated using low current under the low power dissipation condition and the circuit configuration of the RC oscillator can be simplified as well.
However, certain operating characteristics, especially stable oscillation frequency characteristics with varying temperature, are desired as important performance parameters in the RC oscillator. The above-stated patent does not teach in detail on this point. One of the methods to obtain the stable temperature characteristics of the oscillation frequency is to employ a reference power having stable characteristics against the temperature variation. Examples of related art regarding this are disclosed in U.S. Pat. No. 5,828,329 (issued to Lawrence M. Burns) entitled xe2x80x9cADJUSTABLE TEMPERATURE COEFFICIENT CURRENT REFERENCExe2x80x9d and in U.S. Pat. No. 5,672,961 (issued to David W. Entrikin) entitled xe2x80x9cTEMPERATURE STABLIZED CONSTANT FRACTION VOLTAGE CONTROLLED CURRENT SOURCExe2x80x9d. These patents disclose a circuit configuration having the stable temperature characteristics, however, the focus is only pointed on an electric circuit and an example of RC oscillator is not disclosed.
Meanwhile, in the RC oscillator, it is a well-known fact that the oscillation frequency can be variably changed by controlling the value of an external resistor R. A majority of the micropower RC oscillator is composed of one chip. At this time, it is common to arrange the resistor R external to the chip for the oscillation frequency to have an individual relation with chip manufacturing process errors and to control the value of the resistor R. Accordingly, the manufacturing process of the RC oscillator becomes more complicated and the resulting RC oscillator is larger.
The RC oscillator basically includes a resistor R and a capacitance C and also requires a reference power for supplying a power source to the oscillating circuit and a circuit for controlling the voltage level of the oscillation signal. However, the above-mentioned patent references are related to the configuration of a portion of the RC oscillator. An example of a low-power type RC oscillator evaluated to be wholly considered having the advantages such as a low current dissipation, a stable oscillation frequency characteristics with varying temperature, a possibility of integrating in a single chip and an easy controllability of the oscillation frequency is given in U.S. Pat. No. 5,585,765 (issued to Timothy G. O""Shaughnessy) entitled xe2x80x9cLOW POWER RC OSCILLATOR USING A LOW VOLTAGE BIAS CIRCUITxe2x80x9d. The patent suggests an oscillator capable of self-calibrating that compares the oscillator-generated period with an RC time constant by employing an oscillator like a voltage controlled oscillator and controls the frequency of the RC oscillator according to the result compared. According to the patent, the RC oscillator is capable of generating an accurate oscillation frequency by self-calibrating and has a stable oscillation frequency by a self-temperature compensating ability. However, the RC oscillator requires a complicated circuit configuration to provide such functions.
Therefore, it is a first object of the present invention to provide a micropower RC oscillator having a low current dissipation, a variable resistor having complementary characteristics with varying temperature within the chip, and stable oscillation frequency characteristics against the temperature variation, while having a relatively simple circuit configuration.
It is a second object of the present invention to provide an RC oscillating circuit having stable frequency characteristics with respect to the temperature variation by means of variably selecting a plurality of unit resistors which are composed of resistor elements having opposing characteristics against the temperature variation to variably change the resistance value and capable of controlling the oscillation frequency.
To achieve the first object of the present invention, the micropower RC oscillator includes a driving voltage circuit which receives a reference signal having a stable voltage level in view of temperature variation by means of a current source and a load having an opposing temperature varying characteristics and increases a current fan-out capability of the reference signal to generate a driving voltage; and an RC oscillation circuit having an RC circuit including a plurality of inverting circuits connected in series with each other and respectively driven by the driving voltage, a resistor forming a closed loop disposed between an output terminal of a head-inverter connected to an output stage and an input terminal of a tail-inverter, a capacitor C disposed between the resistor and a connecting node of the input terminal of the tail-inverter and the input terminal of the head inverter.
At the output stage, an oscillation signal is oscillated in accordance with a frequency determined by the time constant of the RC circuit outputs.
The resistor R is a variable resistor having a resistance value that is variably set corresponding to external resistance data and the resistor is composed of resistor elements having characteristics opposite the temperature variation according to a predetermined ratio.
Preferably, the resistor further includes a resistance regulator for selecting a resistance value that contributes effectively to determine the oscillation frequency by means of decoding the resistance data.
In some cases, the voltage level of the oscillation frequency obtained from the RC oscillating circuit does not meet the voltage level required from a receiving end. To meet such a case, preferably, the micropower RC oscillator additionally includes an output level shifter for shifting the voltage level of the oscillation signal output from the RC oscillating circuit to a predetermined voltage level.
According to the aforementioned configuration, since the resistor of RC circuit and the driving voltage of RC oscillating circuit have stable characteristics against the temperature variation, variance of the frequency of the oscillation signal is negligible. Also, since the supply voltage required from the oscillating circuit is within the range of about 1.6Vxcx9c3.6V in whole and the current dissipation is under than 6 uA, it has a preferable circuit configuration to design a low current and voltage type RC oscillator.
Meanwhile, to achieve the second object of the present invention, an RC oscillator of the present invention includes a plurality of inverters connected in series and independently driven by the driving voltage and an RC circuit having a variable resistor forming a closed loop disposed between an output terminal of a head-inverter connected to an output stage and an input terminal of a tail-inverter and variably changing a resistance value according to external resistance setting data, and a capacitor C disposed between the input terminal of the head-inverter and the input terminal of the tail-inverter. The frequency of the oscillation signal is determined by the time constant of the RC circuit.
Preferably, the variable resistor includes a number of unit resistors and a resistance regulator for selecting an effective resistance value that contributes to determine an oscillation frequency among the plurality of unit resistors based on the resistance setting data. Each of the unit resistors is preferably comprised of the combination of a P+ diffusion resistor and a poly-silicon resistor having opposing characteristics with the temperature variation. As a composition ratio of the both resistors is preferably to be a ratio that can compensate the difference of the frequency change between the two resistors against the temperature variation, so that the respective unit resistors have a stable characteristics against the temperature variation. The resistance regulator includes a decoder for decoding the resistance setting data and outputting the decoded resistance setting data to a number of output ports, a plurality of inverters, which are connected to the output ports of the decoder, for inverting the output value of the each of output ports and a number of bi-directional transmission gates respectively corresponding to the number of output ports. The resistance regulator includes a resistance selector in which the one end of the number of bi-directional transmission gates are commonly connected with each other and the other end thereof are connected to the one end of the number of unit resistors, and the first control terminal of the bi-directional transmission gates are respectively connected to the output ports of the decoder and the second control terminal thereof are connected to the output terminal of the inverter respectively connected to the output ports of the decoder.
According to the aforementioned configurations, the variance of the oscillation frequency against the temperature variation is relatively small because the characteristics of the resistor with varying temperature are stable. Also, since the resistor can be controlled externally, it is able to easily control the oscillation frequency. Furthermore, the variable resistor circuit can be installed in a single chip with other elements.